(i) Field of the Invention
This invention relates to a polypropylene resin composition of a high syndiotacticity and a preparation process thereof.
(ii) Description of the Prior Art
Although the existence of syndiotactic polypropylenes has been known from old days, those produced by a conventional process, in which propylene is polymerized at a low temperature in the presence of a catalyst comprising a vanadium compound, an ether and an organoaluminum, are of low syndiotacticity and hence can hardly be regarded as characterizing syndiotactic polypropylenes. Copolymers of ethylene and propylene are much less regarded as crystalline polypropylenes. On the other hand, a polypropylene of a good tacticity, e.g., a syndiotactic pentad fraction of more than 0.7, has been discovered for the first time by J. A. Ewen et al. by the use of a catalyst comprising a transition metal compound having an asymmetric ligand and an aluminoxane (J. Am. Chem. Soc., 1988, 110, 6255-6256).
The above-mentioned method of J. A. Ewen et al. exhibits a high catalytic activity per unit amount of the transition metal, thus providing a polymer having a high tacticity and relatively properly balanced physical properties. However, molded articles of the polymer are poor in transparency and unsatisfactory in impact resistance at low temperatures.